An Upper Limit on the Expansion Velocity of Gamma-Ray Burst Candidate SN 2001em

We report on VLBI observations of the Type I b/c supernova 2001em, threeyears after the explosion. It had been suggested that SN 2001em might be ajet-driven gamma ray burst (GRB), with the jet oriented far from the line ofsight so that the GRB would not be visible from earth. To test this conjecture,we determined the size of SN 2001em. It is only marginally resolved at ourresolution of ~0.9 mas. The 3-sigma upper limit on the major axis angular sizeof the radio source was 0.59 mas (FWHM of an elliptical Gaussian),corresponding to a one-sided apparent expansion velocity of 70,000 km/s for adistance of 80 Mpc. No low-brightness jet was seen in our image to a level of4% of the peak brightness. If we assume instead a spherical shell geometrytypical of a supernova, we find the angular radius of SN 2001em was 0.17(-0.10,+0.06) mas, implying an isotropic expansion velocity of 20,000 (-12,000,+7,000) km/s, which is comparable to the expansion velocities of supernovashells. Our observations, therefore, are not consistent with a relativisticallyexpanding radio source in SN 2001em, but are instead consistent with asupernova shell origin of SN 2001's radio emission.Comment: 4 pages, Accepted to Astrophysical Journal Letter